This invention relates to treatment of exhaust gas, in particular from internal combustion engines.
Certain types of such engines, especially lean-burn engines such as diesels, produce exhaust gas containing inter alia nitrogen oxides (NOx) and combustible particulate (soot). The exhaust gases from lean-burn engines are net-oxidising, making the reduction of NOx to N2 more difficult than in a gasoline engine system where approximately equivalent amounts of oxygen and fuel are used. Removal of soot has been made practicable by the Johnson-Matthey Continuously Regenerating Trap (xe2x80x9cCRTxe2x80x9d(trademark)) process (EPA-0341832, U.S. Pat. No. 4,902,487, incorporated herein by reference), in which soot is collected on a filter and oxidised by NO2 that has been enriched in concentration by a preceding step of NO oxidation.
This process, however, is applicable only to exhaust containing, after the preceding oxidation step, at least enough NO2 to oxidise the soot. If an engine is operated with low NOx generation a technical problem arises. The low NOx can result form engine design, expedients such as Exhaust Gas Recycle (EGR), temporaty non-normal load or fuel or temperature conditions, deliberate less-lean engine operation or by injection of reductant into the exhaust gas. The prior processes may convert NOx to N2 to a small extent, for example 3-8%. (Hawker et al. SAE paper 970182).
According to the invention a process for treating combustion exhaust gas containing HC, CO, O2, soot and possibly NOx that comprises the steps of:
i oxidising HC to CO2 and H2O, and NO (if present) to NO2; and
ii oxidising said soot by reaction with NO2:
is characterised by the step of generating NOx by oxidising ammonia (as hereinafter defined) and introducing it into the exhaust gas upstream of step ii. Suitably soot is collected on a filter in step ii, but this is not presently regarded as essential if soot is collected or adheres to the walls or front face of a catalytic component or has an extended residence time in the equipments
Conveniently such ammonia oxidation is effected in step i, using therein one or more catalysts effective alone or together to promote oxidation of HC, CO, NO and ammonia. Preferably HC and CO are oxidised in a first stage within step i and NO is oxidised to NO2 with the fed ammonia in a second stage. (Such a divided step i is the subject of a co-pending GB application 99.13300.1, part of PCT application GB99/03971).
The term xe2x80x98ammoniaxe2x80x99 in relation to added reactants includes also other compounds that produce NOx in the oxidising reaction conditions. Thus for example amino- or amido-compounds can be used, for example hydrazine, urea, guanidine, biuret, cyanuric acid, lower alkylamines such as methylamines, and nitroxy compounds. Ammonia itself or any of these can be injected with other materials, such as: (where appropriate) a non-interfering acid such as carbonic acid; another fluid, suitably providing a liquid solution at ambient temperature that may be oxidisable; steam; air.
The introduction of NOx (xe2x80x9cfirst ammonia additionxe2x80x9d) may be continuous or intermittent.
Especially since the first ammonia addition increases the NOx content of the gas to a level above that due to the engine, the process preferably includes also, after step ii, a step of NOx removal. Several procedures are available for such NOx removal. The procedures are suitably based on catalytic methods, absorption methods, or a combination of both. Continuous catalytic decomposition of NOx to N2 uses a lean-NOx catalyst possibly with adjustment of gas composition to equivalence or rich, or with injection of a NOx-specific reactant such as ammonia (using selective catalytic reduction, SCR). Absorption can be long-term (the absorber is eventually removed and regenerated) or short-term (the absorber is regenerated in-line). Regeneration can be achieved using temperature increase, or using the catalytic methods for decomposition of NOx to N2. The preferred method of NOx removal is use of an absorbent that contains and/or is followed by a catalyst for the NOx-reducing reaction. The addition of a NOx-specific reactant and the use of SCR to regenerate the absorbent is especially preferred. The provision of NOx specific reactant downstream of step ii will be referred to as xe2x80x9csecond ammonia additionxe2x80x9d.
The source of ammonia for the second addition can be selected from the sources specified for the first addition, except for compounds containing oxidisable carbon radicals or reducible nitrogen radicals, since the second addition is to react with NOx to give N2.
Whereas either ammonia addition can be continuous or intermittent, the intermittent/intermittent combination is preferred.
The first ammonia addition may be controlled to provide incomplete reaction or may be in excess, thereby slipping enough ammonia to provide the second ammonia addition.
The composition, temperature and flow-rate of engine exhaust commonly changes with time, for example when establishing steady conditions after start-up, or during changes in power output or chance variation. It is therefore desirable to control the exhaust treatment process to meet such changes. Intermittent first ammonia addition and the resulting NOx addition may be made in response to a signal from sensor means indicating that there is for example
(i) inadequate NOx content in the gas leaving the engine; or
(ii) increase in soot filter pressure drop
which results because an engine has been operating for some time with inadequate NOx or exhaust temperature too low to combust soot on the filter, so that a build-up of soot takes place. The NOx addition is then made until the filter is substantially cleared.
If the oxidation of ammonia is effected over the step i catalyst, and if the engine is operated at varying levels of speed and/or load, such that the exhaust gas temperature varies, the first ammonia addition is made only when the temperature corresponds to formation of NOx from ammonia preferentially to reaction of ammonia with NOx. Since such intermittent ammonia produces an upward step in NOx content, the second ammonia addition will normally coincide in time with it or follow very soon after it. Such addition is controlled in response to detection of NOx exiting the absorber or to the (approaching) end of a time period designed for the absorber. This programmed regeneration period typically lasts 1 to 100 seconds.
The temperatures of the gas at the various stages of the process are controlled as follows:
(i) if the NOx addition is made by oxidising ammonia in step i, the temperature is desirably over 200xc2x0 C., for example in the range 350xc2x0-500xc2x0 C.;
(ii) for regeneration of a NOx absorber, the temperature should be for example 150xc2x0-300xc2x0 C. This is easier to achieve if the second ammonia addition is intermittent, since intermittently higher temperature may be available in high speed engine running or can be obtained for example by oxidation of intermittently provided hydrocarbon in step i or on a pre-catalyst, typically for a time period of the same order as that of the second ammonia addition.
The catalysts and (if used) absorbent are suitably supported on a ceramic or metal honeycomb, coated with a surface-area enlarging washcoat comprising one or more of alumina, zirconia, silicon carbide or other, generally oxidic, material. Coated on the washcoat, in one or more layers, is the active catalytic and/or absorptive material, to be described in more detail below. The honeycomb has typically 50-400 cells per square inch, possibly more, eg up to 1200. The range 200-900 is of general application.
In the oxidation catalyst the active material comprises generally a platinum group metal (xe2x80x9cPGMxe2x80x9d), especially platinum and/or palladium, optionally with other PGMs, eg rhodium, and other catalytic or promoting components. The exact composition and structure of the oxidation catalyst is not critical to operation of the invention, and hence may be varied according to the requirements of the situation. A low temperature light-off formulation is generally preferred. Conventional manufacturing techniques may be used. The catalyst should of course be sized and composed to achieve the necessary conversions, and the design should minimise trapping of soot.
The filter may be any capable of trapping the soot without causing excessive back-pressure. In general, ceramic, sintered metal or woven or non-woven wire filters are usable, and wall-flow honeycomb structures may be particularly suitable. The structural material of the filter is preferably porous ceramic oxide, silicon carbide or sintered metal. A coating such as alumina, and also a catalyst such as La/Cs/V2O5 or PGM may be present. The soot is generally carbon and/or heavy hydrocarbons, and is converted to carbon oxides and H2O. Certain embodiments of this principle are described in the above-referenced patent documents, the teaching of which is incorporated herein by reference.
The absorbent may be selected from:
(a)compounds of alkali metals, alkaline earth metals, rare earth metals and transition metals, capable of forming nitrates and/or nitrites of adequate stability in absorbing conditions and of evolving nitrogen oxides and/or nitrogen in regenerating conditions;
(b) adsorptive materials such as zeolites, carbons and high-area oxides. Compounds (a) may be present (before NOx absorption) as composite oxides, eg of alkaline earth metal and copper such as Baxe2x80x94Cuxe2x80x94O or MnO2xe2x80x94BaCuO2, possibly with added Ce oxide, or Yxe2x80x94Baxe2x80x94Cuxe2x80x94O and Yxe2x80x94Srxe2x80x94Coxe2x80x94O. (The oxides are referred to for simplicity, but in practice hydroxides, carbonates and nitrogen acid salts are present, depending on the temperature and gas composition).
Whichever compounds are used, there may be present also, in and/or following the absorbent, one or more catalytic agents, such as precious metals, effective to promote such reactions as the interchanges of the nitrogen oxides and the action of reductants, and/or of NOx-specific reactants, especially ammonia. If such catalytic agent is present, that is, the absorber is xe2x80x98catalysedxe2x80x99, it may be for example co-precipitated or co-impregnated or co-deposited with the absorbent or present as one or more sandwiched layers or as fine (eg 10-500 microns) particles on or in a layer of absorbent or among particles of absorbent.
The NOx reduction catalysts may be for example: lean-NOx: copper on zeolite (eg ZSM-5) or PGM on alumina or on zeolite (eg mordenite); TWC: platinum and rhodium on alumina; SCR: V2O5/TiO2 or PGM or copper; in each case on a honeycomb structural support.
If ammonia oxidation is effected in a step distinct from step i then the catalyst suitably comprises supported platinum, suitably as in step i. If the fed NOx is to be intermittently supplied by external ammonia oxidation, a trickle of ammonia may be fed between such intermittent supplies, so that the exothermic reaction keeps the external catalyst warm and ready for quick response.
The invention provides a system comprising apparatus integers corresponding to the process as herein defined; and, further, a diesel engine having such an exhaust treatment system. The engine may be for example a heavy duty engine to be used for a duty including idling periods and/or temporary rich running. Another example is a light duty engine, especially if fitted with EGR and/or intended for duty including idling periods. Whichever category of diesel engine is used, it is operated preferably with a fuel containing less than 50 ppm w/w of sulphur compounds, calculated as S. Examples of low-S fuels are xe2x80x98Swedish Environmental Class I dieselxe2x80x99 and xe2x80x98City dieselxe2x80x99. If fuel of more than 50 ppm S is used, the system should include a sulphate trap.
The invention, at least in preferred forms, provides a process, system or engine capable of operation in compliance with projections of European Stage IV, as published in Directive 98/69/EC.